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As it keeps both wholesale and retail prices for Internet service high, Comcast says its pretax operating profit margins are nearly 50 percent. That margin does not include any accounting for the $10 billion it will ultimately cost Comcast and AT&T to update their systems. But the main purpose of that renovation--which involved running high-capacity fiber optic cables into each neighborhood--was to offer hundreds of television channels and interactive services like pay-per-view movies. Internet service uses less than 1 percent of the capacity of these systems.